Feature/Toggle prompt submission visibility

[itinerare]

• Adds a 'hide prompt submissions [never/until prompt ends/forever]' option to prompt creation/editing
• Alters the viewable() submission scope to hide submissions (viewable neither in users' submission logs nor via link) associated with a prompt which either hides submissions forever or (if a prompt has a set future end + is set to hide submissions until end) hides them until the prompt concludes. Staff with the 'manage submissions' permission can, as before, always view all submissions.
• Adds a line to prompt entries in the 'more details' collapse which notes that submissions to the prompt are hidden/hidden until the prompt's end.

Tested locally - Requires migrate